Heavy petroleum fractions, such as vacuum gas oil, or even resids such as atmospheric resid, may be catalytically cracked to lighter and more valuable products, especially gasoline. Catalytically cracked gasoline forms a major part of the gasoline product pool in the United States. The product of catalytic cracking is conventionally recovered and the products fractionated into various fractions such as light gases; naphtha, including light and heavy gasoline; distillate fractions, such as heating oil and Diesel fuel; lube oil base fractions; and heavier fractions.
Sulfur in various forms is commonly found in petroleum and petroleum products either as dissolved free sulfur, hydrogen sulfide, or as organic compounds, such as thiophenes, sulfonic acids, mercaptans, alkylsulfates, and alkyl sulfides. Where a petroleum fraction is being catalytically cracked and contains sulfur, the products of catalytic cracking usually contain sulfur impurities which normally require removal, usually by hydrotreating, in order to comply with the relevant product specifications. Such hydrotreating can be done either before or after catalytic cracking. Because naphtha streams from both catalytic, e.g., FCC, and thermal, e.g.,coking, cracking processes contribute most of the sulfur present in the gasoline pool, reducing the sulfur content of cracked naphthas will be important in order to meet liquid transportation sulfur specifications and emission standards.
The ease of sulfur removal from petroleum and its products is dependent upon the type of sulfur-containing compound. Hydrogen sulfide and mercaptans are relatively easy to remove whereas aromatic sulfur compounds such as thiophenes are more difficult to remove. Sulfur impurities tend to concentrate in the heavy fraction of the gasoline, as noted in U.S. Pat. No. 3,957,625 (Orkin) which proposes a method of removing the sulfur by hydrodesulfurization of the heavy fraction of the catalytically cracked gasoline so as to retain the octane contribution from the olefins which are found mainly in the lighter fraction. In one type of conventional, commercial operation, the heavy gasoline fraction is treated in this way. As an alternative, the selectivity for hydrodesulfurization relative to olefin saturation may be shifted by suitable catalyst selection, for example, by the use of a magnesium oxide support instead of the more conventional alumina.
In the hydrotreating of petroleum fractions, particularly naphthas, and most particularly heavy cracked gasoline, the molecules containing the sulfur atoms are mildly hydrocracked so as to release their sulfur, usually as hydrogen sulfide. After the hydrotreating operation is complete, the product may be fractionated, or even just flashed, to release the hydrogen sulfide and collect the now sweetened gasoline. For naphtha hydrotreating, the naphtha is contacted with a suitable hydrotreating catalyst at elevated temperature and somewhat elevated pressure in the presence of a hydrogen atmosphere. One suitable family of catalysts which has been widely used for this service is a combination of a Group VIII and a Group VI element, such as cobalt and molybdenum, on a suitable substrate, such as alumina.
Naphthas, including light and full range naphthas, may be subjected to catalytically reforming so as to increase their octane numbers by converting at least a portion of the paraffins and cycloparaffins in them to aromatics. Fractions to be fed to catalytic reforming also need to be desulfurized before reforming because reforming catalysts are generally not sulfur tolerant. Thus, naphthas are usually pretreated by hydrotreating to reduce their sulfur content before reforming.
Aromatics are generally the source of high octane number, particularly very high research octane numbers and are therefore desirable components of the gasoline pool. They have, however, been the subject of severe limitations as a gasoline component because of possible adverse effects on the ecology, particularly with reference to benzene. It has therefore become desirable, as far as is feasible, to create a gasoline pool in which the higher octanes are contributed by the olefinic and branched chain paraffinic components, rather than the aromatic components. Light and full range naphthas can contribute substantial volume to the gasoline pool, but without reforming or isomerization they do not generally contribute significantly to higher octane values.
Cracked naphtha, as it comes from the catalytic cracker and without any further treatments such as purifying operations, has a relatively high octane number as a result of the presence of olefinic components. It also has an excellent volumetric yield. As such, cracked gasoline is an excellent contributor to the gasoline pool. It contributes a large quantity of product at a high blending octane number. In some cases, this fraction may contribute as much as up to half the gasoline in the refinery pool. Therefore, it is one of the most desirable components of the gasoline pool, and it should not be lightly tampered with.
Other highly unsaturated fractions boiling in the gasoline boiling range, which are produced in some refineries or petrochemical plants, include pyrolysis gasoline. This is a fraction which is often produced as a by-product in the cracking of petroleum fractions to produce light unsaturates, such as ethylene and propylene. Pyrolysis gasoline may have a very high octane number but is quite unstable in the absence of hydrotreating because, in addition to the desirable olefins boiling in the gasoline boiling range, it also contains a substantial proportion of diolefins, which tend to form gums upon storage or standing.
Cracking of naphtha is a highly useful process to increase the yield of gasoline. However, the cracking process also effects sulfur containing materials and results in a reduction in their molecular weight from a range that is greater than the average molecular weight of the gasoline boiling range fraction into a range that is within the molecular weight range of the gasoline fraction. Much of this gasoline boiling range sulfur is contained in aromatic compounds and, consequently, needs to removed by hydrotreating. However, hydrotreating of any of the sulfur containing cracked fractions which boil in the gasoline boiling range, e.g., FCC, pyrolysis and coker naphtha, causes a reduction in the olefin content, and consequently a reduction in the octane number. Further, as the degree of desulfurization increases, the octane number of the normally liquid gasoline boiling range product decreases. Depending on the conditions of the hydrotreating operation, some of the hydrogen may also cause some hydrocracking or aromatic saturation as well as olefin saturation.
Various proposals have been made for removing sulfur while retaining the more desirable olefins. U.S. Pat. No. 4,049,542 (Gibson), for instance, discloses a process in which a copper catalyst is used to desulfurize an olefinic hydrocarbon feed such as catalytically cracked light naphtha.
Other processes for treating catalytically cracked gasolines have also been proposed in the past. For example, U.S. Pat. No. 3,759,821 (Brennan) discloses a process for upgrading catalytically cracked gasoline by fractionating it into a heavier and a lighter fraction and treating the heavier fraction over a ZSM-5 catalyst, after which the treated fraction is blended back into the lighter fraction. Another process in which the cracked gasoline is fractionated prior to treatment is described in U.S. Pat. No. 4,062,762 (Howard) which discloses a process for desulfurizing naphtha by fractionating the naphtha into three fractions each of which is desulfurized by a different procedure, after which the fractions are recombined.
In any case, regardless of the mechanism by which it happens, the decrease in octane which takes place as a consequence of sulfur removal by hydrotreating creates a tension between the growing need to produce gasoline fuels with higher octane number and--because of current ecological considerations--the need to produce cleaner burning, less polluting fuels, especially low sulfur fuels to avoid poisoning of catalyst converters which would adversely affect hydrocarbon emissions. This inherent tension is yet more marked in the current supply situation for low sulfur, sweet crudes.
A paramount objective of the present invention is to provide a process for reducing the sulfur level in naphtha streams while minimizing product losses in volume and octane number.
A particular objective of the present invention is to provide a process for reducing or lowering the amount of sulfur in naphtha attributable to thiophene or thiophenic compounds.
Yet a further objective of the invention is to provide a process for alkylating thiophenic sulfur compounds in naphtha to allow their subsequent separation from naphtha by fractional distillation with a concomitant reduction sulfur content of gasoline boiling range hydrocarbons.